1. FIELD OF THE INVENTION
This invention relates to active type distance detecting devices, and more particularly to the construction of a light receiving portion of the photosensitive element for detecting the reflection of a projected light spot.
2. Description of the Prior Art
Conventionally, the active type distance detecting device having no moving part includes, for example, the reception of the reflection of a light spot projected from the camera body is received, the received position being related to the distance of an object to be photographed on the basis of the principle of trigonometrical survey, range finding informations thereby being obtained. As an example, U.S. Pat. No. 3,820,129 discloses a distance detecting device wherein a photosensitive element having two independent sensitive regions is used. In the following, its principle is simply explained. FIG. 1 illustrates a photosensitive element 1 of rectangular shape of which the sensitive region is divided into two parts by providing a diagonal insensitive region C of constant width, the areas of the two sensitive regions A and B being exactly equal to each other, and their anodes or cathodes being formed on a common substrate. As shown in FIG. 2, a projected image spot P of round shape, projected by a light projecting element 2 provided in a camera body (not shown), passes through a light protection lens 3 and reflects from an object at a position 4, 5 or 6. The reflection of the projected image spot P is focused on the photosensitive element 1 by a collection lens 7. Because the light projecting element 2 and the photosensitive element 1 are parallel positioned in spacing spaced relation by a constant distance in the direction of the base length, when the object distance varies, the image forming position of the reflection of the projected image spot P on the surface of the photosensitive element moves. Therefore, if this image forming position is detected by some method, it is possible to determine the distance to the object. That is, according to an optical arrangement as shown in FIG. 2, when the object lies at a shorter distance, for example, position 4, the reflection of the projected image spot forms an image in such a position as shown by 1a on the photosensitive element 1. Further, as the object distance increases to positions 5 and 6, the image forming position of the reflection of the projected image spot P moves to 1b and 1c, respectively. As the image forming position of the reflection of the projected image spot P changes through 1a, 1b and 1c, the relative values of the detection output of the sensitive region A and the detection output of the sensitive region B varies with variation of the ratio of the light receiving domain (the term "light receiving domain" herein used means those portions of the sensitive regions which actually receive the reflection of the projected image spot P), or the ratio of the light receiving areas. From these relative values, therefore, the object distance can be determined. Thereby, the value obtained by normalizing the output of the sensitive region A or the output of the sensitive region B by the sum of the outputs of the sensitive regions A and B becomes almost proportional to the axial movement of the focusing lens. Letting the outputs of the sensitive regions A and B be denoted by "a" and "b" respectively, the focusing position of the photographic lens can be controlled by obtaining the value of a/(a+b), or b/(a+b).
And, if the angle of inclination of the aforesaid insensitive region C is determined so that from the prescribed optical condition, a boundary between the shortest and shorter object distances lies at a value for, for example, a/(a+b)=1/4, another boundary between the shorter and intermediate object distances at a value for a/(a+b)=1/2, and still another boundary between the intermediate and farther object distances at a value for a/(a+b)=3/4, it becomes possible to determine a correct position of the photographic lens.
However, in actual practice, with the use of such a system, as the diameter of the image of the reflection of the projected image spot P changes at random, or the optical position deviates, the above-defined object distance boundaries are caused to shift largely. For example, when the starting point of the optical arrangement is taken at an object distance for a/(a+b)=1/2, as shown in FIG. 3(A), on the basis of a certain diameter of the image of the reflection of the projected image spot P, the boundary position (a/(a+b)=1/4) between the shortest and shorter object distances, and the boundary position (a/(a+b)=3/4) between the intermediate and farther object distances can be found to be equal to desired values of object distance. This enables the inclination of the insensitive region C to be set. As shown in FIG. 3(b), however, when the actual diameter of the image of the reflection of the projected image spot becomes larger than that, despite the image of the reflection of the projected image spot P shifting by the same quantity on the surface of the photosensitive element 1, the amount of variations of the outputs "a" and "b" becomes small. As a result, the actual object distance for a/(a+b)=1/4 becomes shorter than the prescribed one, and the actual object distance for a/(a+b)=3/4 conversely becomes farther. Also when the actual diameter of the image of the reflection of the projected image spot P becomes small, the amount of variations of the outputs "a" and "b" becomes large. As a result, the actual object distance for a/(a+b)=1/4 becomes farther than the prescribed one, and the actual object distance for a/(a+b) 3/4 becomes shorter than the prescribed one.